A spectrographic multiplexer having an array of waveguides conventionally comprises a dispersive array of optical waveguides connected to inlet waveguides and to outlet waveguides via two star couplers. The field in an inlet waveguide is reproduced in the plane of the outlet waveguides when the optical path length difference between two adjacent waveguides of the array is equal to an integer number of times the inlet wavelength. In other words, the maximum intensity position depends on the equiphase plane at the outlet of the array of waveguides and thus depends on wavelength. Such a configuration thus makes it possible to separate various wavelengths in space. A variation in wavelength gives rise to a shift in field distribution on the outlet waveguides.
By way of example, such components are used as a 1 to N demultiplexer, as an N to 1 multiplexer, or as an N to N multiplexer with switching.
A particularly advantageous application lies in the field of optical fiber telecommunications, for example in a direct detection receiver circuit in a transmission system. In an optical distribution network, such a component can be used for a plurality of users to separate signals of different wavelengths arriving on a common optical fiber, with each user receiving a respective wavelength. Such a component is also advantageously used in an optical device for interconnection, e.g. between fast electronic "chips" having a high degree of integration, or between computers, or even within a computer.
Usually, the spectral response obtained in an outlet waveguide of such a component corresponds to coupling a Gaussian beam in a Gaussian waveguide and is therefore itself Gaussian.
Nevertheless, a Gaussian spectral response requires wavelengths to be controlled accurately on emission, which makes such a response difficult to use in a system. The existence of small fluctuations in the wavelength at which a laser emits (fluctuations due to temperature) thus make it necessary for such lasers to be servo-controlled, which is difficult and expensive, or to use channels having spectral responses that are wider.
Enlarging spectral response also makes it easier to achieve independence from polarization. The techniques that have been proposed for obtaining independence from polarization and that rely on controlling waveguide geometry continue to deliver poor performance.
By using a flat or square spectral response, the power received on a channel is independent of polarization, even if the two TE and TM peaks are slightly offset (offset associated, for example, with poor control over geometry).
Several techniques have already been proposed for making a spectrograph having an array of waveguides and presenting a spectral response of the square type.
Proposals have thus been made to modify the mode shape of the inlet (or outlet) waveguides. In a phasar, light in the inlet waveguide propagates through the first coupler and spreads laterally. The waveguides of the array intercept the light which then propagates in each of the waveguides, and interferes in the second coupler. The field at the inlet of the outlet waveguides, as created by this interference, reproduces the field at the outlet of the inlet waveguide. Thus, the spectral response in terms of transmission between the inlet and the outlet as a function of wavelength corresponds to the convolution of the inlet mode and the outlet mode.
Thus, if the inlet mode can be considered as being the superposition of two peaks, convolution with a Gaussian outlet mode gives a flattened spectral response.
Thus, "Recent improvements in arrayed waveguide grating dense wavelength division multi/demultiplexers", Hitachi Cable Limited, by H. Uetsuka et al., E.C.I.O. 97, proposes a first solution consisting in using a Y junction at the inlet to the first coupler. Light is shared equally between the two branches of the junction. The inlet mode can thus be considered as being made up of two peaks. The convolution of these two peaks with the outlet waveguide mode is therefore a flattened function.
In "Chirping of a MMI-PHASAR multiplexer for application in multi-wavelength lasers", University of Delph: C.G.P. Herben et al., E.C.I.O. 97, a second solution is proposed consisting in using a multimode interference (MMI) coupler for obtaining a "bi-modal" field having two peaks at the inlet of the first coupler. The spectral response is flattened for the same reason.
Both of those two solutions require an additional object such as a Y junction or an MMI to be included, which goes against reducing volumes within systems. Such devices are also tricky to optimize. In addition, those solutions do not enable highly flattened or "square" spectral responses to be obtained.
To quantify the fact that a spectral response is flattened to a greater or lesser extent, a parameter n is defined as the ratio of spectral width at 1 dB over spectral width at 20 dB.
The coefficient .eta. increases with increasing squareness or flatness of the spectral response. By way of example, this coefficient is 23% for a Gaussian response.
Inlet mode modification by those two techniques, using a Y junction or an MMI, makes it possible to obtain a composite mode having a plurality of Gaussian peaks. The resulting mode thus contains Gaussian "flanks", and so does the resulting spectral response. The coefficient .eta. is therefore limited by the intrinsically Gaussian nature of the modes. With a Y junction, a parameter .eta. is obtained that is equal to 32%, and in the MMI solution, the parameter .eta. is 44%. It is not possible to obtain a coefficient .eta. of close to 90% using those methods.
In "Passband flattening of PHASAR WDM using input and output star couplers designed with two focal points", Corning: D. Trouchet, A. Beguin, H. Boerk, C. Prel, C. Lerminiaux, R. O. Maschmayer, OFC 1997 Technical Digest, p. 302, a technique is proposed that consists in using two focal points in the outlet coupler, enabling the energy to be separated into two Gaussian peaks. The superposition of those peaks convoluted with the outlet mode makes it possible to obtain a spectral response that is flattened.
In French patent application FR-96 11601, a phasar is proposed having two arrays of waveguides of different pitch.
Patent application FR-96 11601 also proposes a phasar in which an inlet waveguide of the inlet coupler or an outlet waveguide of the outlet coupler has a range such that the waveguide, while remaining monomode, presents a two-peak shape.
In all three of the above techniques, the flanks of the spectral response that is obtained are Gaussian. It is therefore not possible to obtain spectral responses that are very square.
In "Arrayed-waveguide grating multiplexer with a flat spectral response", NTT, K. Okamoto and H. Yamada, Optics Letters, January 1995, Vol. 20, No. 1, proposals are made to obtain power distribution in the array that is of the "sinc" or "(sin x)/x" type, by modifying the power distribution at the inlet of the array of waveguides. Since the spectral response is the Fourier transform of this distribution, it is square.
The distribution of power in the array is due to expanding the mode of the inlet waveguide coupled to the waveguides of the array. This expansion is generally modelled by a Fourier transform. Since the waveguides generally used are integrated monomode waveguides having, to a good approximation, a Gaussian mode laterally (so the field after expansion is also Gaussian), it is difficult to obtain "sinc" type power distribution in the array with such waveguides.
To obtain secondary lobes in the array, it is therefore necessary to modify the mode of the inlet waveguide, which must have a "square" shape. It is therefore necessary to modify the shape of this mode, but without using multimode waveguides.
Furthermore, the secondary lobes of the sinc function must have values that are alternately positive and negative. Since power is always positive, it is necessary to introduce a .pi. phase shift in the array at the guides corresponding to secondary lobes of this power distribution.
This phase shifting corresponding to the secondary lobes must be controlled accurately, since a position error for a waveguide relating to the additional phase shifting degrades the spectral response very severely and makes the phasar unusable. It is also necessary to control accurately the positions of the secondary lobes in the array.
It will be understood that this technique which acts on the shape of the inlet power and on the phase shifting in the array of waveguides is difficult to implement, and that poor control of manufacturing parameters gives rise very quickly to degraded component performance.